The average human ear registers sound frequency in a range of 20 Hz, or 20 cycles per second, to 20 kHz, or 20,000 cycles per second. Animal vocalizations range much higher than 20,000 cps and much lower than 20 cps. Research is showing that many familiar mammals speak further up the scale, in the ultrasonic range, and many others sound off much further down, in the infrasonic.
Fly by Night
Bats make high-pitched sounds measured from as low as 10 kHz to above 160 kHz. Humans with very acute hearing can hear part of the lower range of bat sounds, but most of them are beyond us. Bats use these sounds to "echolocate" both prey and obstacles in their flight paths. Using this technique, they can fly safely and accurately in total darkness -- useful if you live in a cave -- as they zero in on tiny insects on the wing or swoop down to skim the surface of water for a quick sip.
Philippine tarsiers (Tarsius syrichta), among the world's tiniest primates, use ultrasound for communication. Their calls have been documented at upwards of 90 kHz. Using this high frequency allows them to pass along information on the whereabouts of their favorite insects without interference from lower-frequency background noise; their calls are also harder for predators to hear and home in on.
Many marine mammals use sound outside the range of human hearing to navigate in water in much the same way that bats fly in air, bouncing calls off obstacles and prey and listening for return vibrations, just as a submarine navigates by sonar. Dolphins are known to use a span of 18 kHz to 23 kHz for these purposes. Some whales can sound off as high as 175 kHz, higher than bats or even glass-shattering sopranos. Humpback whales use sounds as low as 30 Hz, depriving us of the full "basso profundo" effects of their famous songs, since humans hear hardly anything slower than 100 Hz.
How Low Can You Go?
At the bottom of the mammal sound scale are elephant rumbles, at 5 Hz to 20 Hz. These extremely low-frequency sounds allow elephants to communicate over distances as great as 20 miles or more, depending on weather conditions. These low, far-carrying calls are especially important in helping dominant bulls and estruos cows meet and mate. Elephants can make these very low sounds because of their great size and because of some special adaptations of their physical sound-production equipment. One of these is the elephant's trunk, which in a large male can add more than 6 feet to his resonating chamber.
The Expanding Chorus
As the study of animal communications continues, science continues to glean new information about other familiar mammals who have been using these frequencies all along. Tigers, giraffes, hippos and rhinos use infrasonic calls, while rats, mice and sloths sing ultrasoprano.
- Indiana University: What Is Frequency?
- Rice University Houston TeacherTECH Archives: Bat Sounds
- hypertextbook: Frequency of Bat Sonar
- Daily Mail Online: Monkey Known for Being Silent Is Actually a Chatterbox - Using Ultrasound That Predators Can't Hear
- Wild Mammal: Tarsiers -- Communication in the Ultrasound
- Softpedia: Top 7 Ultrasound Emitting Animals